The next generation of optoelectronic and logic devices may be based on spinach, not silicon.

If a team of researchers at Oak Ridge National Laboratory (ORNL) has their way, the next generation of optoelectronic and logic devices may be based on spinach, not silicon. At the APS Centennial meeting in Atlanta, the scientists announced that they have discovered new methods to orient spinach leaf proteins - specifically, the photosynthetic reaction centers that convert electromagnetic (light) energy into stored chemical energy - in such a way as to enable their use for super-high-resolution video imaging, ultrafast switching, logic devices, and solar power generation.

The ORNL work is part of a burgeoning new field known broadly as molecular electronics, in which scientists attempt to use various biomaterials to perform complex functions that are difficult to achieve with other kinds of materials, such as semiconductors. "Natural materials have been optimized for these functions by billions of years of evolution and often perform them better than any human-designed material could," says Paul Kolodner, a researcher with Bell Laboratories/Lucent Technologies, who is seeking to exploit the natural electrochromic properties of the protein bacteriohodopsin for reflective flat panel displays. "Furthermore, organisms manufacture biological materials all by themselves - all we have to do is feed them and harvest the products." In contrast, the production of such devices as integrated circuits requires many time-consuming, high-precision and expensive manufacturing steps. In other work in this area, researchers at the University of Massachusetts, Boston, are investigating the use of chemically stabilized films of bacteriohodopsin in a polymer matrix for all-optical switching and modulation, while at Bell Labs/Lucent Technologies, efforts are geared towards the use of organic materials in smart pixel arrays for use in flat panel displays, and DNA computing (see below).

According to ORNL's Ida Lee, green plant leaves such as spinach contain two pigment protein complexes that perform photosynthetic functions, essentially using the Sun's energy to create plant tissue. The proteins are known as Photosystem I (PSI) and Photosystem II (PSII). The first step was to isolate the proteins from spinach leaves, using a common food processor to chop up the spinach and straining the pulp through a cheesecloth to separate.the juice, after which a high-speed centrifuge process is used to isolate the PSIs. The resulting substance is then tested for purity using a scanning tunneling microscope. Once isolated, the proteins can be rewired to produce fuels such as oxygen or hydrogen by metallocatalysis and photosynthetic water splitting. Isolated PSI reaction centers - naturally occurring photovoltaic and diode structures - can also be used to generate electrical current when provided with an electrical contact, which the researchers achieved by depositing platinum - a good electrical conductor - on one end of the protein, and anchoring it to a gold surface.

Critical to the potential of PSI for biomolecular electronic applications is the ability to arrange the proteins so that all the same ends point in the same direction, known as preferred orientation. The ORNL team accomplishes this by chemically treating the atomically flat gold surface on a mica substrate, achieving the best results with mercaptoacetic acid and mercaptoethanol. The sulfur atom in each chemical binds strongly to gold, and the negatively charged ends selectively bind to the positively charged ends of the PSIs, causing them to point in the same direction, says Lee, adding, "These experiments are the first demonstration that PSI can be isolated, platinized, and selectively oriented by chemical modification of a surface without denaturation."

The ultimate goal, of course, is the feasibility of a "lean clean green machine" that operates without using fossil fuels, according to Marty Goolsby, manager of ORNL's Communications and Public Affairs division. "Such a machine could meet many DOE mission goals by generating electrical power, producing clean hydrogen fuel, and fixing carbon dioxide on a ruthenium film to produce methane fuel and reduce the atmospheric content of greenhouse gases," he says. "It could be a DOE dream machine."